Automatic gearboxes of the automated stagegeared gearbox type are being used more often in heavier vehicles with the increasing development of microprocessor systems. Using a control computer and a number of control devices, for example servomotors, such systems are able to precisely regulate the engine speed, engagement and disengagement of an automatic clutch between the engine and gearbox (and gearbox clutch members in relation to one another) so that smooth gear changes are always achieved at the correct engine speed. One advantage of this type of automatic gearbox, compared to a conventional automatic gearbox made up of planetary gear trains and having a hydrodynamic torque converter on the inlet side, lies partly in the fact that the present system is more simple and robust. Another advantage of the arrangement is that it can be manufactured at substantially lower cost than a conventional automatic gearbox, especially when used in heavy vehicles, and partly in that it affords greater efficiency, especially by way of reduced fuel consumption in the incorporating vehicle.
The development of computer technology has also had an impact on electronic control and feedback systems for a vehicle engine, and as a result, these systems have become more precise, faster and more adaptable to prevailing engine and environmental conditions. Using such technology, substantially the entire combustion process can be precisely controlled according to any operating situation. In this regard, mention should also be made here of various sensors useable for the detection of parameters essential for engine feedback and control.
A manufacturer of internal combustion engines usually optimizes an engine design in order to achieve a certain engine power output, low fuel consumption, good efficiency and high torque. By designing software incorporated into the engine control unit in a certain way, it is possible to precisely determine the appearance of an engine torque and speed curve; that is to say, what maximum torque the engine must be capable of delivering at a certain number of revolutions. This is done, in a supercharged engine (exhaust turbocharger or compressor driven by the crankshaft), for example, by controlling the boost pressure in such a way that a certain maximum torque is obtained at a certain number of revolutions. The design dimensions of the engine hardware naturally imposes purely physical or strength limits on what the engine can, on the whole, deliver. The manufacturer selects a torque and engine speed curve of a certain appearance for a particular engine model, and according to which the engine is allowed to deliver its maximum torque.
If, in driving the vehicle, the engine speed rises or falls to such an extent that the maximum engine torque is insufficient, the vehicle transmission (gearbox) should be shifted up or down so that the engine torque can be better utilized. By means of gearboxes as described above, the gearing between the engine and the vehicle's driving wheels is adjusted so that the engine speed attains a level for which the corresponding maximum torque is sufficient. In this sense, it should be regarded that the engine and the gearbox form the vehicle powertrain.
A stagegeared gearbox usually comprises (includes, but is not necessarily limited to) an input shaft, an intermediate shaft (with at least one toothed gear meshing with a toothed gear on the input shaft), and a main shaft (with toothed gears that mesh with toothed gears on the intermediate shaft). The main shaft is then further connected to an output shaft coupled to the driving wheels, by way of a prop shaft, for example. Each pair of toothed gears has a different gear ratio from another pair of gears in the gearbox. Different transmission ratios are obtained by way of different pairs of gears transmitting the torque from the engine to the driving wheels. Between two interacting and rotating toothed gears in a gearbox, friction losses occur between the teeth of each of the toothed gears which are in engagement.
In some stagegeared gearboxes the highest gear (lowest gear ratio) is a so-called direct gear. This implies that the input shaft and the main shaft (or the output shaft) in the gearbox are directly connected to one another when the direct gear is engaged. This means that the torque is transmitted straight through the gearbox without any gearing. It may alternatively be said that the transmission ratio is 1:1; consequently, no losses occur between meshing gears. An effect of the direct gear is that more fuel-savings is achieved than in the indirect gears, the transmission ratios of which are obtained through the pairs of toothed gears.
Driving a vehicle in a direct gear saves fuel in that friction losses in the gearbox are lower. When the vehicle encounters a sufficiently increased rolling resistance, due to a steeper uphill gradient, for example, or increasing headwind, the vehicle may begin to slow down due to the fact that the maximum torque from the engine is not sufficient to keep the vehicle speed constant.
In order to obtain greater motive force on the vehicle's driving wheels, an automatic stagegeared gearbox will change down to a lower gear (higher gear ratio). Owing to the new higher gearing of the engine torque, the powertrain is aimed to deliver sufficient torque to the driving wheels to be at least capable of maintaining a somewhat slower speed.
The equivalent can be said to occur in the case of brief braking in which the speed reduction does not directly cause down-shift, but the vehicle speed, after braking, is such that the corresponding engine speed and maximum torque with the direct gear engaged are insufficient and the vehicle begins to decelerate and the gearbox will then change down.
While the direct gear can be fuel-saving in this manner, it is usually at the same time also the most powerful from a strength point of view. That is to say, the transmission is capable of transmitting higher torque in the direct gear than in other gears in which the torque is transmitted via the toothing systems on the toothed gears meshing in the gear pair for each particular gear.
U.S. Pat. No. 5,876,302 shows an arrangement in which the engine control unit permits a higher engine output torque when a direct gear in a stagegeared gearbox is engaged. The object according to this arrangement is to utilize the greater strength of the direct gear. As soon as the control unit receives an input signal indicating that a direct gear is engaged, the engine control unit allows a higher maximum permitted drive torque output from the engine.
U.S. Pat. No. 5,679,096 shows an arrangement in which a stagegeared gearbox is equipped with a direct gear and so-called overdrive gears enhanced from the strength standpoint (that is to say overdrive gear in which the transmission ratio between the rotational speed of the gearbox input and output shafts is less than 1:1). In this way, a higher maximum permitted engine output torque can be allowed for the direct gear and the overdrive gears compared to the lower gears. When the control unit receives an input signal indicating that a direct gear or overdrive gear is engaged, the engine control unit allows a higher maximum permitted output drive torque from the engine.
It should be appreciated, however, that driving in a direct gear with an increase in the maximum permitted engine output torque does not necessarily mean that the vehicle will be driven with greater fuel economy than if the vehicle were driven with an indirect gear with a lower maximum permitted engine output torque. Therefore, a need exists to reduce the fuel consumption of a vehicle in which the engine, with a direct gear engaged, can deliver an increased maximum permitted torque. This is the primary object of the invention described below.